添加元素时Arraylist的Arraylist问题

Question

I'm trying to do something equivalent to the following C++ code:

vector <vector<int> > adjlist;
adjlist.resize(maxv+1);
adjlist[n1].push_back(n2);

Where n1,n2 are integers, I've googled a lot and found that Arraylists are the closest match in Java so I tried the following:

List<List<Integer> > adjlist =  new ArrayList<List<Integer> >(maxv);
adjlist.get(n1).add(n2);

but it's not working at all, I know I can build another arraylist let's name it al then the following would work:

adjlist.add(al);

but that won't work for me, I need to add individual integers as I mentioned, any help?

Answer 1

int n = 5;

    List<List<Integer> > list =  new ArrayList<List<Integer> >(n);
    for(int i=0;i<n;i++)
    {
        list.add(new ArrayList<Integer>());
    }

    for(int i=0;i<n;i++)
    {
        for(int j=0;j<10;j++)
        {
            list.get(i).add(j);
        }
    }
    System.out.println(list);

Answer 2

Keep in mind that you can always add something to an Object which itself has been added to something as long as you retain the Object reference . In this case, the object is an ArrayList . So you can do the following:

    int n = 5;
    List<List<Integer>> lists = new ArrayList<>();
    int m = 10;
    for (int i = 0; i < n; i++) {
        List<Integer> temp = new ArrayList<>();
        lists.add(temp);
        for (int j = 0; j < 10; j++) {
            temp.add(j * m);
        }
        m *= 10;
    }

Now print them.

    for (List<Integer> lst : lists) {
        System.out.println(lst);
    }

[0, 10, 20, 30, 40, 50, 60, 70, 80, 90]
[0, 100, 200, 300, 400, 500, 600, 700, 800, 900]
[0, 1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, 9000]
[0, 10000, 20000, 30000, 40000, 50000, 60000, 70000, 80000, 90000]
[0, 100000, 200000, 300000, 400000, 500000, 600000, 700000, 800000, 900000]

Note that I added a multiplier m to show that each list is different.